linux-kernel - Re: [PATCH 1/2] sched/fair: Record the average duration of a task

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Message-ID: <4e28fd17d7a2f7146aae10a76982f0e58b22befb.camel@gmx.de>
Date: Wed, 03 Jul 2024 13:57:19 +0200
From: Mike Galbraith <efault@....de>
To: Raghavendra K T <raghavendra.kt@....com>, Chen Yu <yu.c.chen@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
 Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Tim Chen <tim.c.chen@...el.com>, Yujie Liu
 <yujie.liu@...el.com>, K Prateek Nayak <kprateek.nayak@....com>, "Gautham R
 . Shenoy" <gautham.shenoy@....com>,  Chen Yu <yu.chen.surf@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/fair: Record the average duration of a task

On Wed, 2024-07-03 at 14:04 +0530, Raghavendra K T wrote:
>
>
> On 7/1/2024 8:27 PM, Chen Yu wrote:
> >
> > A thought occurred to me that one possible method to determine if the waker
> > and wakee share data could be to leverage the NUMA balance's numa_group data structure.
> > As numa balance periodically scans the task's VMA space and groups tasks accessing
> > the same physical page into one numa_group, we can infer that if the waker and wakee
> > are within the same numa_group, they are likely to share data, and it might be
> > appropriate to place the wakee on top of the waker.
> >
> > CC Raghavendra here in case he has any insights.
> >
>
> Agree with your thought here,
>
> So I imagine two possible things to explore here.
>
> 1) Use task1, task2 numa_group and check if they belong to same
> numa_group, also check if there is a possibility of M:N relationship
> by checking if t1/t2->numa_group->nr_tasks > 1 etc
>
> 2) Given a VMA we can use vma_numab_state pids_active[] if task1, task2
> (threads) possibly interested in same VMA.
> Latter one looks to be practically difficult because we don't want to
> sweep across VMAs perhaps..

Oooh dear.. as soon as you mention threads, the question of who's
wheelhouse is this in springs to mind, ie should the kernel be
overriding userspace by targeting bits of threaded programs for forced
serialization?

Bah, think I'll just bugger off and let you guys have a go at making
this stacking business do less harm than good.

	-Mike